The present invention relates to a process for the hydrogenation of nitrile functional groups to amine functional groups.
It relates more particularly to a process for the complete or partial hydrogenation of dinitrile compounds to diamine or aminonitrile compounds.
Hydrogenation of dinitriles to the corresponding diamines is a process which has been used for a long time, in particular hydrogenation of adiponitrile to hexamethylenediamine, one of the base materials in the preparation of polyamide-6,6.
An increasing interest has become apparent in recent years in the hydrogenation (also sometimes known as hemihydrogenation) of aliphatic dinitriles to aminonitriles, in particular the hydrogenation of adiponitrile to 6-aminocapronitrile, resulting either directly or via caprolactam in polyamide-6.
Thus, U.S. Pat. No. 5,151,543 discloses a process for the selective hydrogenation of aliphatic dinitriles to the corresponding aminonitriles, at 25-150xc2x0 C. and under a pressure of greater than atmospheric pressure, in the presence of a solvent in a molar excess of at least 2/1 with respect to the dinitrile, the solvent comprising liquid ammonia or an alcohol with 1 to 4 carbon atoms and an inorganic base which is soluble in the said alcohol, in the presence of a Raney catalyst, the aminonitrile obtained being recovered as main product.
Patent WO-A-93/16034 discloses a process for the preparation of 6-aminocapronitrile by hydrogenation of adiponitrile in the presence of an inorganic base, of a transition metal complex, the transition metal being of low valency and chosen from chromium, tungsten, cobalt and iron, and of Raney nickel as catalyst, under hydrogen pressure and at a temperature of 50xc2x0 C. to 90xc2x0 C.
Patent WO-A-96/18603 discloses the hemihydrogenation of aliphatic dinitriles to aminonitriles by hydrogen in the presence of a catalyst based on optionally doped Raney cobalt or nickel and of a strong inorganic base, the starting hydrogenation medium comprising water, aminonitrile and/or diamine which are capable of being formed and unconverted dinitrile.
All these hydrogenation processes result in the desired aminonitrile and are presented as being able to be employed continuously in an industrial plant.
However, the selectivities and the yields of these processes have to be improved to render them more competitive.
One of the aims of the present invention is to provide a process for the hydrogenation of nitrile functional groups in the presence of a catalyst exhibiting an improved yield and an improved selectivity.
To this end, the invention provides a process for the hydrogenation of nitrile functional groups to amine functional groups using hydrogen in the presence of a hydrogenation catalyst and of a strong inorganic base preferably deriving from an alkali metal or alkaline earth metal.
According to the invention, the process comprises a stage of conditioning the catalyst which consists in mixing the hydrogenation catalyst, a predetermined amount of strong inorganic base and a solvent in which the strong inorganic base is not very soluble. According to the invention, the medium comprising a catalyst thus conditioned is fed into the hydrogenation reactor, the hydrogenation reaction being carried out according to the usual conditions or procedures already disclosed in the literature.
The term xe2x80x9chydrogenation catalystxe2x80x9d is understood to mean, in particular and advantageously, Raney metals, such as Raney nickel or Raney cobalt, mixed oxides with a hydrotalcite structure, as disclosed in WO97/10052, but also supported metals, in particular metals from Group VIII of the Periodic Table of the Elements, such as nickel, cobalt, ruthenium or rhodium, deposited on a support, which is generally a metal oxide or active charcoal.
In the case of Raney metals, their instability on contact with the air requires the use of a liquid storage medium. This liquid medium is generally water.
According to the invention, the solvent used exhibits a good affinity for the storage liquid, generally water, thus making it possible to obtain phase separation and formation of a phase comprising the strong inorganic base at a high concentration.
According to a preferred embodiment, the strong inorganic base is added to the said storage medium before the addition of the solvent.
In the case of the other catalysts, which do not require the presence of a storage liquid, it may be worthwhile and advantageous to add water to the mixture.
The term xe2x80x9caffinity between the solvent and the storage liquid or waterxe2x80x9d should be understood as meaning that these compounds are soluble in one another.
Likewise, the term xe2x80x9cnot very solublexe2x80x9d used to characterize the solubility of the strong inorganic base in the solvent should be interpreted as meaning a solubility of less than 3% by weight of the said base in the pure solvent.
According to the invention, the order of addition of the components or the mixture is immaterial.
According to the process of the invention, the presence of the solvent results in a phase separation of the strong inorganic base or of a concentrated solution of strong inorganic base, forming a second liquid phase comprising all or essentially all of the amount of base added to the mixture, this said phase comprising the strong base being and remaining in intimate contact with the catalyst, the first phase being formed by the solvent and the storage liquid and optionally the solvent of the base, if the latter is added in the form of a solution in a solvent, such as water.
Therefore, the catalyst particles come into contact with a concentrated solution of strong inorganic base, allowing the catalyst to be conditioned by attachment or adsorption of the molecules of strong base at the surface of the said catalyst.
The use of a catalyst comprising molecules of strong base at its surface makes it possible to carry out a hydrogenation with an improved yield and selectivity which are reflected in particular by a decrease in the impurities formed, as is illustrated in the examples given below.
The hydrogenation catalyst can advantageously comprise, in addition to the catalytic metal, a doping element chosen from the elements from Groups Ib, IIb, IVb, VIb, VIIb and VIII of the Periodic Table of the Elements, as published in the Handbook of Chemistry and Physics (Weast, 5th edition of 1970-1971), and aluminium, present in particular in Raney metals.
The term xe2x80x9cRaney metalxe2x80x9d is understood as meaning in particular Raney nickel or Raney cobalt.
The strong inorganic bases which are suitable for the invention are alkali metal or alkaline earth metal hydroxides, for example LiOH, NaOH, KOH, RbOH, CsOH, and their mixtures.
According to another characteristic of the invention, the liquid storage medium for the Raney metal is preferably water.
According to one characteristic of the invention, the amount of strong base added in the stage of conditioning the catalyst is between 0.1 mol and 50 mol per kg of catalyst. The ultimate amount of base is determined for each catalyst.
According to a preferred form of the invention, the strong base is added in the conditioning stage in the form of a concentrated solution or in the pure form.
Furthermore, the amount of solvent added depends on the degree of solubility of water or of the storage liquid in this solvent and on the desired level of concentration in the phase comprising the strong base. Advantageously, the ratio by weight of the solvent to the water (or storage liquid) will be at least equal to 1, preferably greater than or equal to 2.
According to the invention, the solvent is chosen from the compounds which have an affinity (solubilizing ability, for example) for water or the storage liquid for the Raney metal and which, in contrast, do not have an affinity (low solubilizing ability) for the strong inorganic base. The concept of insolubility of the strong base in the solvent or more specifically in the liquid phase formed by the solvent and the water or the storage liquid should be understood as meaning a low solubility of the base, for example of less than 1% by weight.
In a preferred embodiment of the invention, the solvent is advantageously an amine, preferably an amine corresponding to that obtained by the hydrogenation reaction, or liquid ammonia, in the case where the hydrogenation is carried out in a liquid ammonia medium. This is because the choice of the solvent should advantageously not allow new substances to be introduced into the hydrogenation medium and thus should make possible easy and inexpensive separation and optionally recycling processes which are thus not very penalizing for the process from a technical and economical viewpoint.
The stage of conditioning the catalyst can be carried out under an inert atmosphere, optionally under a hydrogen atmosphere or under hydrogen pressure.
The process of the invention applies more particularly to the hydrogenation of dinitriles, such as adiponitrile, to diamines, such as hexamethylenediamine (HMD), or to the partial hydrogenation or hemihydrogenation of dinitriles, such as adiponitrile, to aminonitriles, such as aminocapronitrile. The latter reaction is particularly advantageous for the manufacture of lactams, such as xcex5-caprolactam, obtained by cyclizing hydrolysis of the aminonitrile.
Generally, this hemihydrogenation reaction is carried out in the presence of water, which represents between 0.1 and 20% by weight of the reaction medium, or in the presence of another compound, for example liquid ammonia, the concentration of this compound advantageously being less than 50% by weight of the reaction medium.
Thus, in a specific embodiment of a hemihydrogenation, the starting hydrogenation medium comprises water in a proportion of at least 0.5% by weight with respect to all the liquid compounds of the reaction medium. The medium also comprises one or more diamines and/or aminonitriles, capable of being formed from the dinitrile by hydrogenation with hydrogen, and unconverted dinitrile in a proportion, for the combination of these three compounds, of 80% and 99.5% by weight with respect to the combined liquid compounds of the reaction medium.
The aliphatic dinitriles which can be employed in the process of the invention are more particularly the dinitriles of general formula (I):
NCxe2x80x94Rxe2x80x94CNxe2x80x83xe2x80x83(I) 
in which R represents a linear or branched alkylene or alkenylene group having from 1 to 12 carbon atoms.
Use is preferably made, in the process of the invention, of dinitriles of formula (I) in which R represents a linear or branched alkylene radical having from 2 to 6 carbon atoms.
Mention may in particular be made, as examples of such dinitriles, of adiponitrile, methylglutaronitrile, ethylsuccinonitrile, malononitrile, succinonitrile, glutaronitrile and their mixtures, in particular the mixtures of adiponitrile and/or of methylglutaronitrile and/or of ethylsuccinonitrile which can originate from the same process for the synthesis of adiponitrile.
In practice, the case where R=(CH2)4 will be the most frequent as this corresponds to the use of adiponitrile (ADN) in the present process.
It is also possible, in the process of the invention, to add a strong base to the hydrogenation reaction medium which is identical to or different from that used for the conditioning of the catalyst. This strong base is generally an alkali metal or alkaline earth metal hydroxide, carbonate or alkoxide.
The reaction medium has a composition varying according to the type of implementation of the process.
This is because, if the process is carried out batchwise, as is in particular the case in tests on the laboratory scale or for small-scale manufacturing trials, the starting reaction medium will gradually grow richer in aminonitrile and, to a lesser extent, in diamine, whereas the concentration of dinitrile can either decrease, if all or most of the said dinitrile is charged from the beginning of the hemihydrogenation, or can remain relatively constant, if the dinitrile is introduced gradually during the reaction.
In contrast, if the process is carried out continuously, the average composition of the reaction medium reaches values predetermined by the degree of conversion and the selectivities of the reaction.
Water is usually present in an amount of less than or equal to 20%. The water content of the reaction medium is preferably between 0.5% and 15% by weight with respect to the combined liquid constituents of the said medium.
The concentration of the targeted aminonitrile and/or of the corresponding diamine and of the unconverted dinitrile in the reaction medium is generally between 85% and 98% by weight with respect to the combined liquids included in the said reaction medium.
The catalysts used in this hemihydrogenation process can be a Raney nickel or a Raney cobalt comprising, in addition to the nickel or the cobalt and the residual amounts of the metal removed from the starting alloy during the preparation of the catalyst, that is to say generally aluminium, one or more other elements, often known as doping elements, such as, for example, chromium, titanium, molybdenum, copper, tungsten, iron or zinc. Among these doping elements, chromium, copper, titanium, iron and their mixtures are regarded as the most advantageous. These doping elements usually represent, by weight with respect to the weight of nickel or of cobalt, from 0% to 15% and preferably from 0% to 10%.
Use may also advantageously be made of a catalyst based on ruthenium deposited on a support composed of acetylene black. This catalyst can also comprise doping metal elements included in the list mentioned for the Raney metals.
The amount of catalyst employed can vary very widely according in particular to the nature of the catalyst and the method of operation adopted or the reaction conditions chosen. By way of indication, use may be made of 0.5% to 50% by weight of catalyst, expressed as weight of metal with respect to the total weight of the reaction medium, and generally of 1% to 35% by weight.
The process of the invention is generally carried out at a reaction temperature of less than or equal to 150xc2x0 C., preferably of less than or equal to 120xc2x0 C. and more preferably still of less than or equal to 100xc2x0 C.
In concrete terms, this temperature is between ambient temperature (approximately 20xc2x0 C.) and 100xc2x0 C.
Prior to, simultaneously with or subsequent to the heating, the reaction chamber is brought to the appropriate hydrogen pressure, that is to say, in practice, between 1 bar (0.10 MPa) and 100 bar (10 MPa) and preferably between 5 bar (0.5 MPa) and 50 bar (5 MPa).
The other conditions which govern the hydrogenation (continuous or batchwise) in accordance with the invention relate to conventional technical arrangements which are known per se.
Furthermore, these conditions can be modified in order to modify the degree of conversion of the dinitrile to diamine according to whether a high selectivity for aminonitrile is desired or conversely complete hydrogenation of the dinitriles to diamines is desired.
The following examples, given solely by way of indication, illustrate the invention.
In these examples, the following abbreviations may be used:
ADN=adiponitrile
ACN=aminocapronitrile
HMD=hexamethylenediamine
DC=degree of conversion (% by weight of adiponitrile converted)
YD=selectivity with respect to the converted starting substrate (mol % of compound ACN (YDACN) or HMD (YDHMD) obtained with respect to the total amount of ADN converted).